Direct up-converter transmitters are used in various communication devices that employ time division synchronous code division multiple access (TD-SCDMA) and wideband code division multiple access (W-CDMA) modulation schemes. As is unfortunately typical in such transmitters, there is a local oscillator (LO) leakage component present at the output which degrades error vector magnitude (EVM) determination. There are typically two components that contribute to this LO leakage. First, differential signals generated in the baseband stages may be skewed, i.e., are not centered, and, when applied to a differential mixer, a DC offset in the up-converted signal is generated. This is referred to herein as carrier feed-through. The second source, referred to herein as direct LO leakage, is generated in the RF path to the transceiver transmit (Tx) output. DC offset correction between differential signals by onboard circuitry in the baseband circuit may be employed to remove the differential LO leakage component. However, efficient mechanisms for removing the direct LO leakage component, particularly those that are onboard the transmitter, remain elusive. Unfortunately, direct LO leakage is quite dominant at lower output power resulting in a higher error vector magnitude (EVM) at such lower output power.
A DC offset calibration procedure determines the amount of DC offset correction that is required for a given device and is typically performed at the point of manufacture. Often, onboard circuitry is incorporated to assist with the calibration procedure. To determine the optimal correction, one might sweep through possible I and Q DC offset values and monitor the measured LO leakage power relative to the desired output power until the LO leakage power is minimized. The problem with this approach is that it requires numerous measurements, the number of which is proportional to the granularity of I/Q DC offset steps that are swept through. Thus, there is a tradeoff between accuracy and calibration time.
Compounding the calibration time issue is that the sweeping of I/Q DC offset values must be carried out for each of several power levels, typically between −25 dBm and −55 dBm, and for each of several frequency bands. This is due to the fact that the magnitude and phase of the direct LO leakage component are dependent on transmitter gain and LO frequency. Since the overall time required to determine the correct LO leakage offset is a significant factor in manufacturing throughput, sweeping through potential DC offset values for each of several power levels and for each of several LO frequency levels is undesirable. Accordingly, the need is apparent for a direct LO leakage offset correction technique that is both accurate and requires as little calibration time as possible.